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| Mass Concrete - Pour, Temperature Monitoring, Procedure |
Introduction Of Mass Concrete
American Concrete Institute (ASI) defines mass concrete as, For any volume of concrete which we are going to pour, requires arrangement to look after cracks and stresses generated inside concrete due to Heat of hydration is known as mass concrete.(ACI 116R and ACI 301).
Mass concrete is normally used in large structures such as Dams, bridges, Rafts etc. In Mass concrete, the concrete is poured continuously in large quantity, layer by layers for which we take many things under consideration such as the type of cement we are using, type of concrete we are going to use, type of materials which generate low heat of hydration can be used etc. All the things we should work out because the property of cement is generating heat of hydration when react with water, Due to this as the heat of hydration increases, The heat inside the concrete generates which result in generation of cracks inside the concrete resulting in stress and later on decreasing the Strength, Durability and Stability of concrete.
Since main concern of mass concrete is working with temperature control, so the team should be familiar with the temperature to be maintained, temperature to be controlled , And the temperature raise after concreting while setting and hardening of Concrete.
Temperature
The temperature plays a major role in mass concreting, which generates cracks inside concrete resulting in decrease in strength and the durability. In the setting stage of concrete, alot of heat is generated for which we apply curing methods to concrete, But since due to large thickness the curing only decrease the temperature of the upper layers of concrete and the lower layers and the layers in the middle still be at high temperature which results in generating cracks.
Necessity of Temperature control
Concrete tends to show poor thermal conductivity due to which thermal expansion is not uniform in the concrete. Because of this when the tensile load acts on the concrete, the concrete is unable to take the tensile load and later leads to development of cracks inside the concrete because of uneven thermal expansion.
Temperature to be maintained
While pouring the concrete we maintain the temperature between 28°C (82.4°F) to 35°C (95°F). As if the concrete is poured at very low temperature, there will be chances of freezing the water inside it, which results in creation of voids, whereas if concrete is poured at high temperature there are chances of evaporation of water at higher rate, leading in more rapid heat of hydration rate of concrete.
As per American concrete institute (ASI), the maximum temperature the concrete can achieve in its full hydration period is 71°C (160°F) and should not go beyond that else due to the tensile stresses, cracks will generate. Similarly the difference between the core of the concrete and other layer should not be more than 1.7°C which is 35°F. If the difference go beyond that. The more the thermal gradient, the sever the cracks will form.
Precautions to overcome Temperature problems
· Low heat producing materials
1) Use of Pozzolona Cement (PPC)
The cements contains siliceous materials such as fly ash, GGBS upto 30% . This results in generating low heat of hydration. Cement will not gain its strength rapidly but will gain high strength in 28 days of time.
2) Use of Blast Furnace Slag Cement (Highly preffered)
The cement contains 60% of slag, materials like fly ash, GGBS. This highly reduce the setting time of cement resulting in generating low heat of hydration.
3) Retarders can be added
Retarders such as hydroxycarboxylic acids with salts can be added on a dosage of 0.2%- 0.6% , to lower down the hydration and setting time of concrete.
4) Chilled water used
While batching of cement, chilled water at 0°C from chiller plants is used in mass concreting. This makes the aggregate of the concrete cool, The amount of aggregate is high in concrete, which results in showing high significant change.
· Pre-cooling method
1) Use Of liquid Nitrogen
The method tends to show high effect in pre-cooling of the materials inside concrete at the time of batching of mix , but is a costlier technique.
· Post cooling technique
1) Use of Cooling pipes
The cooling pipes are installed in the location of concrete, inside the surface. These cooling pipes have continuous cold water flowing inside them which keeps the concrete cool.
· Insulation of Concrete
1) Concrete is insulated by not letting the moisture content inside to concrete to move out. From this people use many types of curing methods as using epoxy resin chemical compound, Covering the concrete with LDP (low density polymer sheets), Placing a thick coat of 100mm sand on top surface, not removing the formwork from sides etc.
Procedure & Things to be considered in Mass Concrete
When we are about to do mass concreting alot of things have to be considered to execute the work. This work varies from each location based on the duration and the area of the work, the mass concrete is going to be executed.
1) Checklist Checks
All the check lists should be checked and signed with the respective authority who give permission or the approval for execution of work before starting the mass concrete. There are certain checklists for
· Work Inspection Report
· Checklist For Quality
· Checklist For Safety
· Checklist For Reinforcement
· Checklist For Formwork
· Concrete Pour card
Once you have all of them signed, you can start the work.
2) Arrangement Of Labour
As labours are the most important people in construction industry, So we need arrange the working schedule for them. Who will be there for night hours and who all will be for day time. If the concrete is going to happen for more than a day than the food for them have to be considered.
3) Arrangement of Extra JCB's and Dozzers
As the weight of tansit mixture with concrete is close to 20 thousand kg's. There use to be chances of deterioration of the surface where the movement of these mixers happen. Just to overcome the issue and keep the movement uniform, Dozzers or JCB's should be available. There are even chances when the machinery to fill up the batching plants get breakdown so even this should be considered.
4) Concrete to poured in Layers
Concrete should be poured in layers to 300mm , We will not have joints and the uniformity will be maintained.
5) Mode Of Communication
There should be proper communication between the people who are inside the mass concreting area and the people who are managing outside.
6) Stand-by Pump or Boom placer
A stand-by machine should be available if case of breakdown.
7) Ambulance for Emergency
Always to be at safer side, a mode for emergency transportation should be available in sites.
8) Proper Compaction
Proper supervision of compaction should be done and has to be maintained as it is directly proportional to the strength of concrete.
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